msan_builder: Inject MSan flags in wrapper if they don't exist.
msan_builder: Inject MSan flags in wrapper if they don't exist.

Some packages build test binaries that don't use DEB_C(XX)FLAGS and fail
during linking.

googlefonts/oss-fuzz,googlefonts/oss-fuzz,googlefonts/oss-fuzz,google/oss-fuzz,robertswiecki/oss-fuzz,googlefonts/oss-fuzz,robertswiecki/oss-fuzz,googlefonts/oss-fuzz,robertswiecki/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz,robertswiecki/oss-fuzz,FeliciaLim/oss-fuzz,googlefonts/oss-fuzz,google/oss-fuzz,google/oss-fuzz,robertswiecki/oss-fuzz,googlefonts/oss-fuzz,FeliciaLim/oss-fuzz,googlefonts/oss-fuzz,skia-dev/oss-fuzz,FeliciaLim/oss-fuzz,google/oss-fuzz,robertswiecki/oss-fuzz,google/oss-fuzz,FeliciaLim/oss-fuzz,robertswiecki/oss-fuzz,FeliciaLim/oss-fuzz,googlefonts/oss-fuzz,google/oss-fuzz,skia-dev/oss-fuzz,google/oss-fuzz,robertswiecki/oss-fuzz,google/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,robertswiecki/oss-fuzz,google/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,skia-dev/oss-fuzz,robertswiecki/oss-fuzz,google/oss-fuzz,googlefonts/oss-fuzz,robertswiecki/oss-fuzz